Readonly for immutable reactive data in Vue

Introduction

Readonly makes reactive data unchangeable. It helps keep data safe from accidental changes.

When you want to share data but prevent others from changing it.

When you have a constant configuration object in your app.

When you want to protect state that should not be modified after setup.

When you pass data to child components and want to avoid mutations.

When debugging to ensure some data stays fixed.

Syntax

Vue

import { reactive, readonly } from 'vue';

const original = reactive({ count: 0 });
const locked = readonly(original);

readonly() wraps reactive data to make it immutable.

Trying to change locked will not update the data and will warn in dev mode.

Examples

This example shows how to create a readonly version of reactive state.

Vue

import { reactive, readonly } from 'vue';

const state = reactive({ name: 'Alice' });
const frozenState = readonly(state);

// frozenState.name = 'Bob'; // This will not change the value

Readonly can also be used on plain objects to prevent changes.

Vue

import { readonly } from 'vue';

const config = readonly({ theme: 'dark', language: 'en' });

// config.theme = 'light'; // This change is ignored

Sample Program

This Vue component shows a count that is wrapped in readonly. The button tries to increment the count by changing the original reactive state. Direct changes to the readonly object are ignored.

Vue

<template>
  <div>
    <p>Count: {{ locked.count }}</p>
    <button @click="increment">Increment</button>
  </div>
</template>

<script setup>
import { reactive, readonly } from 'vue';

const state = reactive({ count: 0 });
const locked = readonly(state);

function increment() {
  // Trying to change locked.count will not work
  // locked.count++ // This will warn and not update

  // Change original reactive state instead
  state.count++;
}
</script>

Important Notes

Readonly only prevents changes on the wrapped object, not on the original reactive source.

Readonly is useful to protect data passed to child components.

In development mode, Vue warns if you try to mutate readonly data.

Summary

Readonly makes reactive data immutable to prevent accidental changes.

Use it when you want to share data safely without allowing modifications.

Readonly objects reflect changes from the original reactive source but cannot be changed directly.

Practice

1. What does the readonly function do in Vue's reactivity system?

easy

A. It creates an immutable reactive object that cannot be changed directly.

B. It makes a reactive object completely non-reactive and static.

C. It clones the reactive object and allows changes only on the clone.

D. It converts a reactive object into a plain JavaScript object.

Solution

1. Step 1: Understand the purpose of readonly

   The readonly function wraps a reactive object to prevent direct modifications, making it immutable.

2. Step 2: Compare with other behaviors

   Unlike cloning or removing reactivity, readonly keeps the object reactive but disallows changes.

3. Final Answer:

   It creates an immutable reactive object that cannot be changed directly. -> Option A

4. Quick Check:

   Readonly = Immutable reactive object [OK]

Hint: Readonly means no direct changes allowed on reactive data [OK]

Common Mistakes:

• Thinking readonly removes reactivity completely
• Assuming readonly clones data instead of wrapping
• Believing readonly allows changes on the wrapped object

2. Which of the following is the correct way to create a readonly reactive object in Vue?

easy

A. const data = readonly({ count: 0 })

B. const data = readonly(reactive({ count: 0 }))

C. const data = reactive(readonly({ count: 0 }))

D. const data = reactive({ count: 0 }).readonly()

Solution

1. Step 1: Understand the correct usage order

   You first create a reactive object, then wrap it with readonly to make it immutable.

2. Step 2: Evaluate each option

   const data = readonly(reactive({ count: 0 })) correctly wraps a reactive object with readonly. const data = reactive(readonly({ count: 0 })) tries to make a readonly object reactive, which is invalid. const data = readonly({ count: 0 }) skips reactive, so no reactivity. const data = reactive({ count: 0 }).readonly() uses a non-existent method.

3. Final Answer:

   const data = readonly(reactive({ count: 0 })) -> Option B

4. Quick Check:

   Wrap reactive with readonly = const data = readonly(reactive({ count: 0 })) [OK]

Hint: Wrap reactive() inside readonly() to make immutable reactive [OK]

Common Mistakes:

• Calling readonly before reactive
• Skipping reactive and only using readonly
• Using non-existent readonly method on reactive

3. Given the code below, what will be logged to the console?

const state = reactive({ count: 1 });
const readOnlyState = readonly(state);

readOnlyState.count = 5;
console.log(readOnlyState.count);
state.count = 10;
console.log(readOnlyState.count);

medium

A. Error on assignment, then 10

B. 5 and 10

C. 1 and 1

D. 1 and 10

Solution

1. Step 1: Understand readonly behavior on assignment

   Assigning to readOnlyState.count causes a warning in development mode, but the change is ignored and value remains 1.

2. Step 2: Check reactive source update reflection

   First console.log(readOnlyState.count) outputs 1. Changing state.count to 10 updates the source, so second log is 10.

3. Final Answer:

   1 and 10 -> Option D

4. Quick Check:

   Readonly warns/ignores direct change, reflects source updates [OK]

Hint: Readonly blocks direct changes but shows source updates [OK]

Common Mistakes:

• Expecting readonly to allow direct assignment
• Thinking readonly copies data instead of reflecting source
• Ignoring error/warning on assignment to readonly

4. Identify the error in the following Vue code using readonly:

import { reactive, readonly } from 'vue';
const data = readonly({ count: 0 });
data.count = 1;
console.log(data.count);

medium

A. You cannot assign to a readonly object directly.

B. You must use ref() instead of reactive() for readonly.

C. Readonly does not exist in Vue's API.

D. You must wrap a reactive object with readonly, not a plain object.

Solution

1. Step 1: Check how readonly is used

   The code creates a readonly proxy but then attempts data.count = 1.

2. Step 2: Understand readonly requirements

   Readonly prevents direct mutations, causing a development warning and ignoring the assignment.

3. Final Answer:

   You cannot assign to a readonly object directly. -> Option A

4. Quick Check:

   Cannot mutate readonly objects [OK]

Hint: Readonly objects cannot be mutated directly [OK]

Common Mistakes:

• Attempting to mutate readonly objects directly
• Assuming readonly works without reactive
• Confusing readonly with ref

5. You have a reactive object state shared across components. You want to expose it as readonly to prevent accidental changes but still reflect updates. Which approach correctly achieves this and why?

const state = reactive({ user: { name: 'Alice' }, count: 0 });

// Options:
// A) const safeState = readonly(state);
// B) const safeState = reactive(state);
// C) const safeState = readonly({ ...state });
// D) const safeState = reactive(readonly(state));

hard

A. Create a shallow copy and make it readonly, but it won't reflect nested updates.

B. Wrap the reactive state with readonly to prevent direct changes but reflect updates.

C. Wrap the state again with reactive, which allows changes and duplicates reactivity.

D. Wrap readonly with reactive, which is invalid and causes errors.

Solution

1. Step 1: Analyze each option's effect

   Wrap the reactive state with readonly to prevent direct changes but reflect updates. correctly wraps the reactive object with readonly, making it immutable but still reactive to source changes. Wrap the state again with reactive, which allows changes and duplicates reactivity. duplicates reactivity and allows changes. Create a shallow copy and make it readonly, but it won't reflect nested updates. creates a shallow copy, losing nested reactivity. Wrap readonly with reactive, which is invalid and causes errors. is invalid usage.

2. Step 2: Confirm correct approach for safe sharing

   Readonly wrapping reactive is the recommended way to share data safely without allowing modifications but reflecting updates.

3. Final Answer:

   Wrap the reactive state with readonly to prevent direct changes but reflect updates. -> Option C

4. Quick Check:

   Readonly wraps reactive for safe, immutable sharing [OK]

Hint: Wrap reactive with readonly to share immutable reactive data [OK]

Common Mistakes:

• Using shallow copies losing nested reactivity
• Wrapping readonly inside reactive incorrectly
• Duplicating reactivity causing unexpected mutations